1. Technical Field
This invention relates generally to the field of renewable energy power management. More specifically, this invention relates to the power production, power conversion, and power management of DC energy sources systems.
2. Description of the Related Art
Coal-burning energy produces some of the highest greenhouse-gas emissions of any of the fuels in widespread use. The United States currently uses coal-burning fuel to provide about half of the country's electric power. The United States is continually striving to find cheap and efficient ways to generate its own clean energy in an effort to improve the environment and achieve energy independence.
The shift to more energy-efficient policies can also create new jobs. In California, for example, nearly one and a half million jobs have been created between 1977 and 2007 as a result of energy-efficiency policies. The state's policies improved employee compensation by $44.6 billion.
Solar power is one of the cleanest sources of energy available. Sunlight is captured from the sun in the form of electromagnetic radiation and generated into a direct current (DC) using photovoltaic (PV) cells. The PV cells are made of semiconductors, e.g. silicon and are fabricated in the form of semiconductor arrays, films, inks, or other materials. The individual PV cells can be aggregated, interconnected together, and then packaged into solar panels of some size and shape and within a rugged, environmentally sealed enclosure that is suitable for physical mounting and/or installation on residences, businesses, earth-mounted poles, vehicles, roof-tops, and other locations.
The DC has a current (I) and voltage (V). The relationship between the currents produced by a solar panel or series-connected group of panels and the output voltage may be plotted or graphed on an XY axis as a family of IV curves. The solar panel output current I bears a direct relationship to the spectral power density or level of sunlight (spectral irradiance) illuminating the panel at a given time, and may change dramatically relative to small changes in irradiance. In the typical case where some number of such solar panels are series connected, the solar panel with the lowest level of current flow will dictate or set the current flowing throughout the series circuit. Panels connected in series can lose up to 60% of their energy as a result of being limited by the worst-performing panel.
FIG. 1 illustrates one type of problem associated with series-connected solar panels that are limited by the solar panel with the lowest level of current flow. On a sunny day, all the panels may receive the same level of sunlight. If there are any clouds 100 in the sky, however, they may partially obscure a panel 110. So even though some of the panels may be receiving almost all sunlight 120, because the system is series connected 130, the current flow is limited by the worst performing panel 110.
The optimal power of the solar panel array is obtained by incorporating a maximum power point tracking (MPPT) algorithm to optimize the overall power available for harvesting to maintain the power output at the maximum level possible for a given system or string current. Usually the electronics and any software necessary to implement this MPPT function are incorporated into the implementation of the system's DC-to-AC conversion function (DC-to-AC inverter) in grid-connected PV systems or as a component of a storage battery charging and control system for off-grid solar applications. Global MPPT algorithms provide only the average operating point of the total string, not the maximum. An optimized system provides per panel MPPT functionality to account for individual panel optimum operating points as well as variations in panel operating characteristics.
String inverters must be able to accommodate strings of varying numbers of interconnected panels and a wide variety of panel types. Because of these variations, a traditional DC-to-AC inverter used in a series-connection system is subjected to high stress and heat levels resulting in a one percent failure rate within the first six months.
Various methods have been implemented to maximize the energy output. In U.S. Pat. No. 7,158,395, for example, an outer voltage feedback loop was developed to track trends in increasing power sources and adjust the MPPT algorithm accordingly. This approach suffers from a limitation on overall or maximum system power that may be harvested at any point in time due to the series connection of panels, and the requirement that the current flowing through such a series circuit cannot be any greater than that produced by the panel with the smallest output current.
U.S. Publication Number 2008/0097655 discusses calculating a separate MPPT for each solar panel to optimize power production. [0026] The panels supply power to the bus separately. [0026] Information about each panel is transmitted on top of the bus to a management unit, which is connected to a network using TCP/IP protocol. [0020]-[0021] The management unit provides monitoring and control for system components. [0020]
The design of series-connected panel systems is time consuming. All panels must be from the same manufacturer and be of the same model or power rating, multiple strings of series-connected panels must be of the same length or contain the same number of panels, panels with different orientation to the sun must be treated as separate subsystems, and add-ons to an existing installation are treated as additions of a completely separate subsystem.
The maintenance of series-connected panel systems is also time-consuming. When a system is connected in series, any defects in one of the panels will cause the entire system to fail. This is the same defect that occurs in a string of Christmas lights when one of the lights breaks. To locate the defective panel, a technician must test the panels separately. As a result, the cost of hiring a technician to visit the site and locate the defective panel is prohibitively expensive.